1. Technical Field:
The invention relates to a process for improving oil recovery from a permeability-damaged subterranean sandstone formation and more particularly to a process for restoring permeability to a formation which has been damaged by encroaching fluids and thereafter for stabilizing clays and other fine particles present in the formation to improve oil recovery therefrom.
2. Description of Related Art:
Aqueous fluids flowing through or otherwise encroaching into a subterranean oil-bearing sandstone formation containing clays and other fine particles often result in reduced permeability and fluid flow and subsequent reduced oil production or fluid injection in the formation. Aqueous fluids can encroach into a subterranean formation from a variety of sources, such as an underlying aquifer, a secondary or tertiary oil recovery flood or treatment fluids utilized in the well.
At least two types of damage have been shown to occur when aqueous fluids encroach into a formation. The first type of damage is permeability reduction due to the mobilization, migration, and plugging of the formation matrix by fine particles. The encroaching fluid chemically or mechanically detaches the fine particles from the pore bodies of the sandstone matrix as the fluid flows through the matrix. The fluid entrains the fine particles and conveys them to permeability-reducing positions in the pore throats of the matrix where the fine particles bridge and inhibit flow of other desirable fluids through the matrix.
The phenomenon of fine particle migration is discussed in the literature. See Muecke, T. W., "Formation Fines and Factors Controlling Their Movement in Porous Media," Journal of Petroleum Technology, February 1979, page 144; and Gabriel, G. A. and Inamdar, G. R., "An Experimental Investigation of Fines Migration in Porous Media," paper SPE 12168 presented at the 58th Annual Technical Conference, San Francisco, Calif., Oct. 5-8, 1983.
A second type of damage results from the swelling of clays, such as montmorillonite, which also causes permeability reduction in the formation. With regard to swelling, fresh water is generally the most damaging encroaching fluid because a substantial difference usually exists between the ionic content of fresh water and the connate water.
A number of brines have been investigated which stabilize clays and other fines against migration or swelling. The brines either result in minimal damage when they contact the formation or reduce the risk of future damage when fresh water subsequently contacts the formation. See Grim, R. E., Applied Clay Mineralogy, McGraw Hill Book Company, Inc. N.Y., 1962, beginning at page 298; Smith, C. F., et al, "Potassium, calcium treatments inhibit clay swelling," The Oil and Gas Journal, November 30, 1964, p. 80; U.S. Pat. No. 4,280,560 to Sydansk; and U.S. Pat. No. 4,572,297 to Thigpen, J. R. et al.
The above-cited treatment processes have been shown to effectively stabilize clays and other fine particles in a formation and reduce the risk of future formation damage. However, an effective remedial treatment is needed to restore the permeability of formations which have already been damaged by encroaching fluids and to reduce the likelihood of subsequent permeability reduction caused by continued fluid encroachment.